Sub-models, fvOptions: Removed 'active' switch

It is better to not select and instantiate a model, fvOption etc. than to create
it and set it inactive as the creation process requires reading of settings,
parameters, fields etc. with all the associated specification and storage
without being used.  Also the incomplete implementation added a lot of
complexity in the low-level operation of models introducing a significant
maintenance overhead and development overhead for new models.

tutorials/compressible/rhoPimpleFoam/RAS/mixerVessel2D/constant/MRFProperties

@@ -18,7 +18,6 @@ FoamFile
 MRF1
 {
     cellZone    rotor;
-    active      yes;
 
     // Fixed patches (by default they 'move' with the MRF zone)
     nonRotatingPatches ();

tutorials/compressible/rhoSimpleFoam/angledDuctExplicitFixedCoeff/constant/fvOptions

@@ -24,7 +24,6 @@ porosity
         selectionMode   cellZone;
         cellZone        porosity;
         type            fixedCoeff;
-        active          yes;
 
         fixedCoeffCoeffs
         {